When machining is performed using a machine such as a machine tool or a laser machine, control is performed to cause the position of a tool with respect to a work piece to move along a designated route. This control is called track control. In general, the track control is performed by performing servo control to cause actual positions of movable axes of the machine to follow position commands for the movable axes calculated from the commanded route.
As a problem in performing the track control, an actual track deviates from the commanded route because of, for example, response delays of the movable axes to a control system. Usually, the control is performed for each of the movable axes of the machine. Therefore, because of an error due to, for example, the response delays of the movable axes to the control system, servo system responses of the movable axes lag behind position commands. When a moving direction of the commanded route does not change like a straight line, even if the axes move with delay, a track of the servo system response does not deviate from the commanded route. That is, although an error appears in a tangential direction of the commanded route, an error in a normal direction of the commanded route does not appear. On the other hand, when a moving direction of the commanded route changes like a curve, a corner shape, or the like, an error appears in the normal direction of the commanded route because of delays of the servo control systems of the axes. In the following explanation, in errors of servo system response positions with respect to a position command, components in the tangential direction of the commanded route are referred to as follow-up errors and components in the normal direction of the commanded route are referred to as track errors. In general, the track errors are undesirable because a machining shape does not coincide with an original shape when the track errors occur.
As means for suppressing the track errors, Patent Literature 1 discloses a method of calculating, on the basis of a machining shape recognized by looking ahead a program, optimum feed speed for suppressing an error to a fixed value or less, calculating an error amount that occurs when machining is performed at this speed, and adding a correction vector for canceling the error to an original commanded position to correct the commanded position. The direction of the correction vector is a direction perpendicular to a moving direction (a normal direction). The length of the correction vector is a value obtained by multiplying normal direction acceleration (a value obtained by dividing a square of speed by a curvature radius) with a predetermined coefficient.
Non Patent Literature 1 discloses a method of decomposing a response error of a servo system into a component in a tangential direction and a component in a normal direction of a commanded route and performing feedback control using a control input obtained by multiplying the components in the respective directions with individual gains to thereby independently control a follow-up error and a track error.